Not to be a dick but if the shoulders on the PRL are really 18.75" that is nowhere near a 36 and not even a 38 IMO.

Your not being a dick.
With the P2P at 20" makes the chest 40 - 2 for slim fit(38) or -4(36) for regular fit.
The shoulders are larger than a typical wool suit jacket, but not odd. Shoulder measurements do vary greatly with thicker wools and fabrics like corduroy. I have a 40L Harris Tweed with a shoulder measurements of less than 17" and a Harris Tweed 38L with a shoulder measurement of 18.5"+. I have another 38L older(at least a decade) PLPL slim fit corduroy jacket that should have wide shoulder measurements maybe 19.5".

Edit- Just pulled the RLPL from the garage. It measures just shy of 19.25".

Edit 2- I just realized I haven't worn the RLPL in maybe a year. I will probably sell it this week.